The present invention relates to a new and improved construction of a bone implant for a prosthesis, particularly for a joint or joint member of the human locomotor, especially walking system.
The present invention also relates to a new and improved construction of a tool for preparing a bone especially for bonding agent-free insertion of a prosthesis bone implant having a shaft with a shaft body of a predetermined shape extending between a metaphysis-associated proximal end and a diaphysis-associated distal end of the shaft body.
In its more particular aspects, the present invention specifically relates to a new and improved construction of a bone implant for a prosthesis, specifically an endoprosthesis, particularly for a joint or joint member of the human locomotor, especially walking system and which bone implant contains a shaft which is to be inserted into a cavity formed in a bone, preferably a long bone. The shaft has a total cross-sectional area which increases from its diaphysis-associated distal end toward its metaphysis-associated proximal end. Preferably in the region of the metaphyseal or proximal end, the shaft possesses an intermediate piece or member at which a joint or joint member is arranged or arrangeable. The intermediate piece or member is formed integrally with or separately from the shaft.
A large number of such implantable prostheses, particularly for hip joints, have become known. For the shaft to be anchored in the bone there have been proposed various biocompatible materials, i.e. materials which are compatible with both tissue and body fluids and which are not corrodable by the tissue and the body fluid, such as in particular metals, for example, high alloyed steels, special alloys, ceramics, fiber-plastic composites, wood or the like. Despite all the diversity of construction, the known shafts or anchoring members for such prostheses generally include an oblong shaft body which is inserted into the bone. A joint member, for example, a joint cup, a joint plane, a joint sphere or condyle or a joint roll or trochlea is arranged at the proximal end of the shaft either directly or by means of a connecting or intermediate piece or member provided to maintain predetermined positional angles and spacings. The connecting or intermediate piece or member frequently is formed in one piece with the shaft with which the related joint member is connectable preferably in a detachable manner. The materials of the joint member and of the prosthesis shaft are not identical in most of the cases due to their different functions. Most of the oblong joint prosthesis shafts for insertion particularly into long bones, are constructed with a predetermined shape such that the cross-section decreases from the proximal end region to the distal end region thus enabling an adaptation to natural conditions such as in particular the shape of the cavity formed in the bone. The material removal during preparation of the cavity for receiving the implant shaft is thereby minimized.
For the purpose of anchoring implants in a bone there have become known either the employment of bonding agents, for example, formed on the basis of plastic materials or a bonding agent-free anchoring technique. By means of this direct anchoring technique there can be avoided the problems resulting from heat development and tissue incompatibility of the bonding agent. It is intended that, after insertion of the shaft or shaft member, if possible, no longer period of time has to be spent for incorporation or settling of such shaft or shaft member into the bone and that a secure or stable connection is provided between the shaft and the bone immediately after insertion of the bone implant.
The commercially available bone implant shafts used in practice normally do not have a rotationally symmetric cross-section. In order to provide an anchoring which is secured against rotation, the conventional bone implant shafts possess a multi-angular or multi-corner, preferably rectangular cross-section with rounded edges and with longitudinal grooves, recesses, openings or the like. For adaptation to the bone, many prior art constructions contain a longitudinal curvature. The cavity for receiving the shaft is substantially prepared in such a manner, that a rasp, a reamer or the like which is formed substantially in correspondence to the shape of the shaft to be implanted, is oscillatingly moved, for example, by means of a pneumatic drive, and driven into the bone toward the diaphysis. Although already in use for a long time, this technique does not permit preparing cavities whose walls precisely come into snug contact with the outer or bone contact surface of the shaft over large areas or the entire surface area of such walls. On the contrary, these known implant shafts of various shapes contact the wall of the cavity only at a few locations and over relatively small areas. However, at these very locations the mechanical forces are introduced via zones of relatively small surface area and thus at high pressure load or point load. Such pressure point loads or peaks, however, are known to cause a disturbance in the biological equilibrium in the bone and the bone accordingly reacts such that there results precisely at these locations of increased pressure load a degradation of the bone substance and thus a weakening of the cortex which leads to a loosening of the shaft fit.
Furthermore, there is known, for example, from German Patent Publication No. 2,049,111, a prosthesis member containing a short conical implant shaft with a substantially smooth surface. According to this publication, an adaptation of the shaft modulus of elasticity to that of the bone is intended to be achieved by means of an appropriate mechanism. The construction shown in this publication cannot achieve a secure anchoring against loosening and rotation and cannot be loaded for a longer period of time.